1. Field of the Invention
The present invention relates to seismic energy sources, and in particular to those adapted for nondestructive generation of seismic energy in applications where size, space, and power limitations are critical, such as in a downhole environment. The present invention has also demonstrated utility in other environments, as will hereinafter be demonstrated.
2. Description of the Prior Art
During exploratory oil and gas drilling, it is obviously desirable to ascertain as much information about the formations surrounding the wellbore as possible, in order to identify and accurately locate producing zones and potential producing zones. Well logs are often employed to determine resistivity, conductivity and other characteristics of formations.
In addition to logs, it is also known to utilize a geophone or geophones disposed in a wellbore at various depths to receive seismic energy generated by sources on the surface of the earth. The seismic energy, which is in the form of waves, refracts and reflects as it travels through the encountered formations until it is received by the geophones. Raw data from the geophones is subsequently processed by computer into usable forms for determining formation characteristics. One major disadvantage associated with surface sources is the alteration of the source energy in the weathering layer or LVL on the way to the geophones.
Currently, the above described technique is used extensively in conjunction with "Vertical Seismic Profiling" or VSP, wherein a geophone is lowered in stages to various depths in the wellbore, while seismic energy is generated on the surface, at various locations (offsets) which are remote from the wellbore. The data thus generated is generally processed to obtain useful data in manners well known to the art. It may be readily appreciated that the relocation of both geophones and seismic sources is both costly and time-consuming and that, in populous areas, numerous or often-moved surface sources may be impractical.
To overcome the forgoing limitations of VSP, "Reverse Vertical Seismic Profiling," (RVSP) has been developed as an alternative. In RVSP, a seismic source is placed in a wellbore and an array of geophones is disposed on the surface. The use of a nondestructive downhole source having repetitive energy generation capability is critical for utilization with rapid, repetitive shots and a suitable source of this type must also be usable without causing damage to the well in a cased or uncased hole. This technique also demonstrates the advantage of location below the weathering layer or LVL, and is therefore not subject to the above-mentioned attenuation prior to reflection from and refraction by deeper formations of interest.
Ideally, it is desirable to utilize various types of seismic energy waves, such as P-waves, S.sub.v -waves, and S.sub.h -waves in formation analysis, in order to take advantage of the current state of information processing. This is particularly true in crosswell tomography (also known as cross-bores by surveying) wherein a source is placed in one wellbore and geophones in an adjacent wellbore.
Several attempts at the creation of suitable downhole sources have been made in the recent past. See, for example, U.S. Pat. Nos. 4,702,343 and 4,715,470, wherein, respectively, downhole seismic energy is generated utilizing a servo-controlled hydraulic pumping mechanism or a linear electromagnetic actuator to drive a reactive mass. Seismic energy is imparted to the walls of the wellbore in these applications via a clamping mechanism. Pneumatic downhole seismic sources have also been attempted, as noted by H.C. Hardee, "Downhole Periodic Seismic Sources", Geophysical Prospecting 31, 57-71 (1983). All of the foregoing devices have the disadvantages of being relatively complex mechanically, difficult to package in a compact form suitable for deployment in small wellbores, and of requiring a significant input of energy in order to generate an acceptably strong seismic energy wave.
In a somewhat related field, both magnetostrictive and piezoelectric expansion phenomena have been suggested at the basis for downhole energy waves utilized for acoustic well logging. See, for example, U.S. Pat. Nos. 4,649,525, 4,682,308, and 4,700,803. Piezoelectric bender-type vibrational transducers are also known, such as that disclosed in U.S. Pat. No. 4,525,645. That stacked piezoelectric elements or wafers may be employed to generate significant longitudinal forces is also recognized in U.S. Pat. Nos. 4,753,507 and 4,674,907. The major problems, however, with using either directly applied piezoelectric or magnetostrictive forces to generate energy waves is associated with the relatively small length changes in either type of materials. In other words, while significant forces are possible, significant displacements to move a wave-generating element for seismic purposes are not. In addition, magnetostrictive devices, like their hydraulic or electromagnetic counterparts require unacceptably high input power levels for downhole application.